Hi Brian,
On Dec 8, 2009, at 8:39 PM, Brian Aker wrote:
There is no SELECT in the list, but maybe this is correct. I am
just thinking allowed...
We have 3 types of statements:
DML update statements: INSERT, UPDATE, DELETE
DML read-only statements: SELECT.
DDL statements: CREATE TABLE, ALTER TABLE, etc.
This right here is the heart of the differences. STATEMENTs are not
really what you want. You want to know the sort of action, aka read/
write/reformat...
The problem is, the startStatement() call will need to pass the
statement type and, in the case of DDL, more details (like before and
after image) in a GPB.
Because the engine will have to decide how to handle the statement.
Do you see another possibility here?
And assuming we have 2 sets of calls:
- beginTransaction, commitTransaction/rollbackTransaction
- startStatement, endStatement
We could say, all types of statements require a beginTransaction()
and a startStatement() (and the corresponding endStatement() and
commitTransaction/rollbackTransaction()).
But I don't think this is absolutely correct:
* DML update statements require both beginTransaction() and a
startStatement().
* DML read-only statements only require a beginTransaction() call
because a SELECT does not need a statement level transaction
(because they cannot be rolled back).
* And DDL statements only require a startStatement() because it is
up to the engine to decide if this can be done within a transaction
or not.
For example if beginTransaction() is called before startStatement()
then engines that do not handle DDL in transactions should return
an error. In addition, if a engine does atomic DDL, then it can use
the startStatement() to begin a transaction.
With these calls the engine will have most of the information it
needs.
There is some additional information which should be provided when
a cursor is used:
For example, PBXT needs to know:
- which columns will be accessed (an optimization so that not all
need to be loaded),
- whether rows retrieved will be updated or deleted,
- if the rows need to be locked (as in SELECT FOR UPDATE).
Toru, what's your opinion?
-jay
And this is how the engine would handle "ADD INDEX", or "ENCRYPT
TABLE":
startStatement("ENCRYPT TABLE", "t1") --> return: use custom method
doTableOperation("ENCRYPT TABLE", "t1")
endStatement()
The engine can write table operations to its transaction log, and
in this way it could ensure that the entire ALTER TABLE statement
is atomic.
On Dec 7, 2009, at 4:10 PM, Jay Pipes wrote:
Paul McCullagh wrote:
Hi Toru,
On Dec 7, 2009, at 3:31 AM, Toru Maesaka wrote:
Great to hear another use-case where knowing a statement type in
advance is useful :)
Yes, generally I need to know the following:
- If I have a update type statement (i.e. whether the statement
modifies rows).
- Whether I need a table lock (examples: ALTER TABLE, TRUNCATE,
CHECK).
But, Paul, doesn't this depend on the engine itself? I mean, some
engines can do (some types of) ALTER TABLE without taking a
table lock.
So, is this request really for whether the kernel thinks a table-
level
lock is necessary, or is it really just for a descriptor of the
statement type?
And, if it really does just boil down to the statement type,
then how do
we deal with the reality that Brian speaks about -- that
statement type
will be pluggable, and how do we deal with future statement
types for
pluggable engines?
Is a reasonable solution to pass to engines a sort of "statement
traits"? So, instead of passing ALTER_TABLE, CREATE_TABLE,
UPDATE,
DELETE, etc, we instead pass a std::bitset<> (or uint64_t for C
folks)
containing traits of the statement such as:
MODIFIES_DATA
MODIFIES_DEFINITION
etc, etc
And then to deal with transaction locking concerns, just add a
method to Cursor:
void Cursor::setTransactionIsolationLevel(enum enum_tx_isolation);
Cheers!
Jay
- If we have a SELECT FOR UPDATE.
I was talking to Toru about this, and another possibility is
that we have statements declare a needed "lock type" that any
plugin could then query. I outlined the solution for Toru,
but I don't know if he has written the patch yet :)
I've taken notes from our discussion the other day. I'm
planning on
working on it when I finish testing through my current
progress of
BlitzDB.
Great! :)
For now, I'm happy with Jay's advise of using
current_session().
Cheers,
Toru
On Sat, Dec 5, 2009 at 5:59 AM, Brian Aker <[email protected]>
wrote:
Hi!
On Dec 4, 2009, at 3:12 AM, Paul McCullagh wrote:
If we have a startStatement() call, then it could be used in
place of beginAlter(), assuming we can determine the
statement type, and the tables involved.
The problem with relying on statement type is that at some
point statement type will be pluggable... which means you
would constantly need to update your engine for new statements.
Yuck!
I was talking to Toru about this, and another possibility is
that we have statements declare a needed "lock type" that any
plugin could then query. I outlined the solution for Toru,
but I don't know if he has written the patch yet :)
Then, when a handle is returned to the pool it is deleted,
instead of adding it back to the pool.
BTW very soon engines will own their Cursor objects and will
be free to reuse them.
The locking thread waits until all handles are returned and
deleted before it can proceed. The lock on the pool then
prevents a new table handle from being created while the
locking thread is busy.
Either way, it would be good if Drizzle closes all handlers/
cursors before a table is deleted or renamed.
I would say that long term this will be optional, based on
what the engine requires.
OK, this make things a lot simpler! Indeed, if we don't need
to support LOCK TABLE then external_lock() can be removed
altogether.
Tried removing the external_lock() right now and seeing if
any issues pop up?
Cheers,
-Brian
--
Paul McCullagh
PrimeBase Technologies
www.primebase.org
www.blobstreaming.org
pbxt.blogspot.com
--
Paul McCullagh
PrimeBase Technologies
www.primebase.org
www.blobstreaming.org
pbxt.blogspot.com
--
Paul McCullagh
PrimeBase Technologies
www.primebase.org
www.blobstreaming.org
pbxt.blogspot.com
--
Paul McCullagh
PrimeBase Technologies
www.primebase.org
www.blobstreaming.org
pbxt.blogspot.com
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